Helicopters are known that essentially comprise a fuselage defining a cabin for the crew at the front, a main rotor mounted on the top of a central portion of the fuselage and able to generate the lift and thrust, respectively necessary for lift and directional flight of the helicopter, and an anti-torque rotor projecting sideways from a tail fin of the helicopter.
In greater detail, the main rotor comprises a drive shaft and a plurality of blades hinged on the drive shaft through the interposition of a hub.
The helicopter also comprises at least one engine, a transmission unit interposed between the engine and the drive shaft, and a constraint device securing the fuselage to a support body of the drive shaft and the transmission unit. In practice, the fuselage appears “suspended” from the above-mentioned support body by means of the constraint device.
During the helicopter's normal operation, the engine exerts drive torque on the transmission unit. According to the law of action-reaction, reaction torque is discharged onto the support body and from the latter it is discharged onto the fuselage through the constraint device. This reaction torque is balanced by the counter-torque exerted by the tail rotor on the fuselage.
The constraint device inevitably transmits vibration and noise to the fuselage, which reach the cabin and impair the comfort of the crew.
The constraint device is, in a known manner, formed by a crosspiece, which comprises:                a flange bolted to a casing of the transmission unit and defining an opening; and        a pair of appendages, projecting sideways from respective ends opposite to each other of the flange and connected to the fuselage.        
In order to reduce the transmission of the aforementioned vibration and noise to the cabin, a European patent application published as EP-A-2179922 in the name of the applicant proposed:                constraining each appendage to the fuselage by a pair of respective constraining elements; and        interposing a damper assembly between each constraining element and the respective appendage.        
More in particular, each damper assembly comprises an alternating plurality of layers made of an elastomeric material and layers made of a metallic material whereas the flange is made of metallic material.
Although curbing the transmission of noise and vibration to the fuselage, the solution described in the aforementioned patent application EP-A-2179922 has certain margins for improvement.
In particular, the layers of elastomeric material in the damper assemblies must be sufficiently rigid to be able to support the static load constituted by the reaction torque, which is transmitted from the support body of the transmission unit to the fuselage and the cabin.
The damper assemblies of constraint devices of known type can also contain a small amount of elastomeric material, substantially due to the fact that each damper assembly is interposed between a side of an associated appendage and the fuselage.
It follows that, due to the aforementioned design constraints, it is not possible to size the layers made of elastomeric material in an optimal manner to limit the transmission of noise and/or vibration to the cabin.
More specifically, the applicant has observed that, due to the aforementioned design constraints, the constraint device described in patent application EP-A-2179922 has margins for improvement with respect to the frequency bandwidth in which the transmission of vibration and/or noise is effectively contained.
In particular, the effectiveness of the constraint device described in patent application EP-A-2179922 is particularly susceptible to improvement for frequencies in the order of 1000 Hz, which correspond to the transmission of particularly irritating noise for the occupants of the cabin.
Thus, a need is felt within the industry to have constraint devices that are able to sustain the static load caused by the reaction torque and are optimally sizable for limiting the transmission of noise and/or vibration to the cabin and to the fuselage.
Furthermore, considerable surface areas of the layers of elastomeric material in the aforementioned damper assemblies are exposed to the outside environment, and are therefore subject to significant aging and attack by external agents.
Moreover, the layers of elastomeric material are subjected to static load generated by the reaction torque, which thus gives rise to significant fatigue.
It follows that it is necessary to periodically inspect the plates of elastomeric material or protect them with a protective material.
Thus, a need is felt within the industry to limit the requirement for periodically inspecting and/or protecting the layers of elastomeric material.
U.S. Pat. No. 3,920,202 describes a constraint device, which comprises a substantially circular flange connected to the fuselage via blocks formed by an alternating plurality of layers of a metallic material and layers of an elastomeric material.
U.S. Pat. No. 4,111,386 describes a vibration insulation system for vibrations transmitted to the fuselage by the rotor comprising a plurality of support elements, each of which in turn comprises an alternating plurality of layers of a metallic material and layers of an elastomeric material.
U.S. Pat. No. 4,014,484 discloses a crossbar comprising a central arm fitted to a shaft of the gearbox and a pair of appendages fitted to respective arms of the fuselage.
EP-1918198 discloses a bearing hanger assembly, which includes a planar damper element fitted to a mounting block and defining an opening engaged by a shaft, mainly with the purpose of achieving an easy alignment of the shaft relative to the mounting block.
In greater detail, the planar damper element is made from PTFE filled with a relative small percentage of graphite.